Should I tell on a cheating spouse?

[NessaVanessa]

Went out Friday night with some friends to a club in London. I have a friend called J - Not childhood friend, but we are friendly enough, we know each other as our DC are going to the same school. J has a spouse - M - who I saw in the same club with a small group, don't know if it's friends or coworkers, doesn't matter. M did not see me. Most of their group left after a drink but I did see long, handsy snogging between M and the only other person that stayed. At one point I thought they left because they disappeared but then walked back from where the toilets are. Ew. I am not saying something happened in there, but I wouldn't be surprised either.
I know J loves M and it would be heartbreaking to learn. They have two kids, I thought they are super in love. So I don't know what to do.
Should I tell J what I've seen M doing? I took a few blurry videos from a distance. You can imagine it's not great quality, but I think M is recognisable if you are familiar.
What would you do?
(edit: Name changed)

Oh i just assumed it was the man, sorry. Im not sure then. He or she might think you had a malicious interest ie to split them up because you fancied him.

I was the wife at home with the children. When I was told my life fell apart but I now see that I had to be told as my children and I were living a lie, and the that lie was doing us a lot of damage. The wife deserves to know who she is living with.

I would tell her

him.
The cheater is the wife.

Sorry yes tell him - as a friend, absolutely. Why wouldn’t you?

It's very easy to set up a throwaway anon email. Set one up and send the videos to J. Then wait to hear from J about what M's been up to.